The present invention concerns a phase change optical recording medium having a recording layer of changing phase between a crystalline state and an amorphous state in accordance with the intensity of an irradiation beam and, in particular, it relates to a phase change optical recording medium capable of making an initialization process unnecessary, a manufacturing method thereof and a recording method thereon.
In recent years, study and development have been made vigorously on optical information recording media as means for recording, reading and erasing an enormous amount of information. Particularly, so-called phase change optical disks conducting recording/erasing of information by utilizing reversible phase change of a recording layer between two crystalline and amorphous states have been considered promising since they have an advantage that new information can be recorded simultaneously while erasing old information (hereinafter referred to as xe2x80x9coverwritingxe2x80x9d) by merely changing the power of a laser beam.
As the recording material for the phase change optical disk capable of overwriting, chalcogen alloys such as Inxe2x80x94Se series alloys having low melting temperature and high absorption efficiency of laser beam (refer to Appl. Phys. Lett. Vol. 50, p 667, 1987) or Inxe2x80x94Sbxe2x80x94Te (refer to Appl. Phys. Lett. Vol. 50, p 16, 1987) and Gexe2x80x94Texe2x80x94Sb alloy (refer to Japanese Patent Unexamined Publication Sho 62-53886) have been used mainly.
On the other hand, in a case of actually conducting recording/erasing by using the chalcogen alloys, a dielectric layer comprising at least one material selected from oxides, carbides, fluorides, sulfides and nitrides of metals or semimetals is usually disposed to either one or both of the surfaces of just beneath and just on the recording layer in order to prevent deformation of a substrate, oxidation of the recording layer, material migration along grooves or deformation of the recording layer due to heat upon recording/erasing.
Then, the phase change optical disk of a three layered or four layered structure having, on a transparent substrate, a recording layer comprising a chalcogen alloy, a dielectric layer disposed just beneath and/or just on the recording layer and a reflection layer also serving as a cooling layer (made of Al alloy or the like) disposed to the recording layer on the side opposite to the substrate is predominant, since this is desirable in view of recording/erasing characteristics.
In a usual phase change optical disk, the material of the recording layer is heated to a temperature higher than the melting point by irradiating a laser beam at a recording power to a recording layer and then quenched; so that the layer turned into an amorphous state with the result that a recording mark is formed. While the material of the recording layer is crystallized with the result that the recording mark is erased by irradiating a laser beam at an erasing power to heat the material to a temperature higher than the crystallizing temperature, followed by gradual cooling.
The phase change optical disk described above is manufactured by forming thin films constituting each of layers successively to a substrate, for example, by a sputtering method or a vapor deposition method. Since the recording layer just after the deposition is in the amorphous state, the disk is usually supplied after irradiating a laser beam to crystallize the entire surface. The process is generally called an initialization process.
The phase change optical disk of the three layered or four layered structure described above has a relation of Rc greater than Ra assuming the reflectivity when the recording layer is in a crystalline state as Rc and the reflectivity when the layer is in the amorphous state as Ra. The reflectivity when the recording layer is in the amorphous state is not a sufficient value for stable focusing and tracking by a usual driving apparatus. Then, a sufficient reflectivity can be obtained by turning the recording layer into the crystalline state by applying an initialization process.
However, the initialization process requires a time which it is about one minute and less than one minute in order to initialize the entire optical disk of 120 mm diameter even by a laser beam irradiation method which is most efficient, so the step is attributable to the increased cost in the manufacture of disks. That is, considering a time necessary for processing one optical disk in each of manufacturing steps for the optical disk (cycle time), the time required for the initialization process is longer compared with a step for molding a substrate or a step of depositing films. Accordingly, when the cycle time in the film deposition step is, for example, 8 sec, at least 6 to 7 sets of initialization apparatus which is extremely expensive are required in order to eliminate the time loss upon transfer to the initialization process. As a result, the manufacturing cost for the optical disks is increased by the application of the initialization process.
On the other hand, Japanese Patent Unexamined Publication Hei 7-78354 (EP 642123 A1) and Japanese Patent Unexamined Publication Hei 8-63781 disclose phase change optical disks in which a relation between the reflectivity (Rc) when the recording layer is in the crystalline state and the reflectivity (Ra) when the layer is an amorphous state is: Rc less than Ra. Japanese Patent Unexamined Publication Hei 7-105574 discloses a phase change optical disk in which a relation between the light absorptivity (Ac) when the recording layer is in the crystalline state and the light absorptivity (Aa) when the layer is in the amorphous state is: Ac greater than Aa (that is: Rc less than Ra).
Among them, Japanese Patent Unexamined Publication Hei 7-78354 and Hei 7-105574 describe that Rc less than Ra or Ac greater than Aa can be attained by disposing a metal layer or a light absorption layer between a recording layer and a substrate. Further, Japanese Patent Unexamined Publication Hei 8-63781 describes that Rc less than Ra can be attained by properly selecting the film thickness of a protection layer formed between a recording layer and a substrate.
Further, when setting the relation of the reflectivity as Rc less than Ra, since the relation between the light absorptivity (Ac) when the recording layer is in the crystalline state and the light absorptivity (Aa) when the layer is in the amorphous state becomes Ac greater than Aa, distortion of the shape of an amorphous mark upon overwriting can be suppressed. This can decrease jitters contained in read light to obtain high recording/reading characteristic in mark edge recording capable of high density recording.
As described above, a sufficient reflectivity can be obtained even with no initialization process and high recording/reading characteristics can be obtained in the mark edge recording by a phase change optical disk with the relation of the reflectivity being Rc less than Ra. However, it has been found by the study of the present inventors that when overwriting is conducted with no initialization process in this phase change optical disk, C/N (carrier to noise ratio) upon first recording is lower than C/N upon overwriting.
A subject of the present invention is to obtain high recording characteristic already from first recording even when recording is conducted without conducting the initialization process in a phase change optical disk with a relation of the reflectivity being Rc less than Ra.
In order to solve the foregoing subject, the present invention provide a phase change optical recording medium having at least a substrate and a recording layer formed on one side thereof and changing phase between a crystalline state and an amorphous state in accordance with the intensity of an irradiation beam, and the reflectivity (of said medium) when the recording layer is in the crystalline state is made lower than the reflectivity when said layer in the amorphous state, wherein the recording layer is formed in a stable amorphous state for the entire surface at the time of this deposition.
That is, the phase change optical recording medium according to the present invention includes both of a rewritable phase change optical recording medium having a recording layer changing phase reversibly between a crystalline state and an amorphous state and a write-once phase change optical recording medium capable of recording only for once having a recording layer changing phase in at least one direction between a crystalline state and an amorphous state.
Referring to the phase change between the crystalline state and the amorphous state, a graph shown in FIG. 1 is contained in xe2x80x9cFoundation of Amorphous Semiconductorxe2x80x9d (1982), p 23 published by OHM Co. The graph shows a relationship between the arrangement of atoms (degree of freedom) and a free energy in a thin film occurring phase change between the crystalline state and the amorphous state.
As can been seen from the figure, the free energy is minimized in a crystalline state (C) and a plurality of states where the free energy takes relatively shallow minimal values (A1, A2, A3,- - - ) are present in an amorphous state (A). Then, it is considered that the thin film in each of the amorphous states can override a crest of an activation energy by a small energy applied from the outside in the form of heat and light and transfer to the state of other minimal value in adjacent therewith.
Among the amorphous states, an amorphous state (A1) which is nearest to the crystalline state in view of the arrangement and can reach the crystalline state (C) by merely overriding one crest of the activation energy is defined as a stable amorphous state, while other amorphous states (A2, A3,- - - ) than the above are defined as quasi-stable amorphous states in the present invention. That is, upon transfer from the quasi-stable amorphous state (A2, A3, - - - ) to the crystalline state (C), it is necessary to override plural crests of activation energy. Therefore, a thin film in the quasi-stable amorphous state is difficult to turn into the crystalline state compared with the thin film in the stable amorphous state.
In an existent phase change optical recording medium with a relation of the reflectivity being Rc less than Ra, since the temperature of the substrate is not controlled particularly upon deposition of each of the layers including the recording layer and it remains at a relatively low temperature (for example, the temperature is 20xc2x0 C. or more and less than 35xc2x0 C.), it is considered that the recording layer is in the quasi-stable amorphous state. That is, when recording is conducted without the initialization process, the first recording is conducted to the recording layer in the quasi-stable amorphous state. Therefore, a portion that should be crystallized is not crystallized sufficiently. Then, at or after the second recording in a case of the rewritable type, since the amorphous portion of the recording layer has been turned into the stable amorphous state before this recording by the first recording, the portion to be crystallized is sufficiently crystallized. As a result, this leads to a phenomenon that C/N upon recording is low in a case of the write-once type, and C/N upon first recording is lower than C/N upon overriding in a case of the rewritable type.
On the contrary, in the phase change optical recording medium of the present invention in which the relation of the reflectivity is: Rc less than Ra, since the recording layer is formed in the stable amorphous state for the entire surface at the time of this deposition, first recording is conducted to the recording layer in the stable amorphous state even without conducting the initialization process. Accordingly, a portion that should be crystallized is sufficiently crystallized already from the first recording. As a result, high C/N is obtained in the recording in the case of the write-once type, while high C/N is obtained both in the first recording and at or after the second recording in the case of the rewritable type.
As described above, since the recording layer is deposited into the stable amorphous state in the phase change optical recording medium of the present invention, high C/N can be obtained already from the first recording if recording is conducted even without the initialization process. Therefore, the medium can be supplied without applying the initialization process and, as a result, it is possible to greatly improve the productivity and decrease the production cost.
Further, in the phase change optical recording medium according to the present invention, since it is no use to intentionally lower the high reflectivity (Ra) before recording to the low reflectivity Rc by the initialization process, it is preferred to conduct recording without previously crystallizing the entire surface (that is, without initialization process). Since this retains the high reflectivity before recording, inspection before supplying and various inspections after deposition of films can be applied stably.
Further, as a method of recording an the phase change optical recording medium of the present invention, if a method which the recording marks were made in the amorphous state and portions other than the recording marks were made in the crystalline state is employed, since this is the same recording method as that for the existent phase change optical recording medium supplied after applying the initialization process, it provides a merit that existent recording apparatus can be used as they are.
In addition, since the relation for the reflectivity is Rc less than Ra in the phase change optical recording medium according to the present invention, if the non-data area (the region other than data region, such as an index area) is kept always in the amorphous state by making only the reading beam irradiate to the non-data region upon recording, the non-data area can be kept always at a high reflectivity.
The method of judging as to whether the deposited recording layer is in the stable amorphous state or in the quasi-stable amorphous state can include, for example, the following two methods.
The first method is a method of overwriting at an optimal power (both for the recording power and the erasing power), measuring a noise level (N1) after the first recording and a noise level (Nn) at or after the second recording (for example, second or tenth) and judging based on the difference between both of the measured values (N1 Nn). It can be judged that the deposited recording layer is in the stable amorphous state if an absolute value (N1xe2x88x92Nn) is within a predetermined value and that the layer is in the quasi-stable amorphous state if the absolute value is larger than the predetermined value. As the predetermined value, 6 dB or 3 dB can be mentioned for example.
The second method is method of measuring a temperature-reflectivity curve of the deposited recording layer by a DRS (Dynamic Reflectivity Spectroscopy) and judging based on the chart. The DRS method is a method of dynamically measuring change of the reflectivity while elevating the temperature of the film after deposition at a constant rate. According to this method, since the change of the reflectivity upon phase change from the amorphous state to the crystalline state can be measured usually, the temperature of crystallization and activation energy for crystallization can be recognized.
Since the optical constant is different between the amorphous state and the crystalline state even for recording layers of an identical composition, the reflectivity changes upon phase change from the amorphous state to the crystalline state. Further, since the optical constant usually differs somewhat also between the stable amorphous state and the quasi-stable amorphous state, change of reflectivity can often be measured also upon change from the quasi-stable amorphous state to the amorphous state.
FIG. 2 shows a temperature-reflectivity curve when a recording layer of a composition is deposited in a quasi-stable amorphous state and FIG. 3 shows a temperature-reflectivity curve when a recording layer having the same composition is deposited in a stable amorphous state. Both of them are temperature-reflectivity curves obtained at an identical temperature elevation rate.
In FIG. 2, since the recording layer is deposited into the quasi-stable amorphous state, it does not directly changes from the reflectivity in the amorphous state (Ra) to the reflectivity (Rc) in the crystalline state, but there exists a temperature region that shows a reflectivity (RX) smaller than the reflectivity (Ra) in the amorphous state at a position slightly lower than the temperature at which the reflectivity changes to that in the crystalline state. It can be seen by the presence of the temperature region showing the reflectivity RX that the recording layer changes from the quasi-stable amorphous state by way of the stable amorphous state to the crystalline state if the layer is deposited to the quasi-stable amorphous state. However, depending on the composition of the recording layer or the film constitution as the recording medium, the temperature region showing the reflectivity RX may be extremely small and can not be determined easily. On the other hand, in FIG. 3, since the recording layer is deposited in the stable amorphous state, it changes from the reflectivity (Ra) in the amorphous state directly to the reflectivity (Rc) in the crystalline state in the vicinity of the crystallizing temperature.
FIG. 4 and FIG. 5 are copy images of TEM (Transmission Electron Microscopic) photographs and electron beam diffraction patterns (upper right portion) showing thin film of Ge32Te68 deposited in the amorphous state. The thin film shown in FIG. 4 is partially applied with a treatment to form a stable amorphous state but the thin film shown in FIG. 5 is not applied with such treatment.
That is, the thin film shown in FIG. 5 is the thin film of Ge32Te68 deposited on a substrate formed with grooves for guiding a laser beam without heating the substrate and it can be seen that the film is in the amorphous state based on the electron beam diffraction pattern. Further, since a fine structure is observed from the TEM photograph, it is considered that the thin film is deposited in the quasi-stable amorphous state.
The TEM photograph in FIG. 4 shows a state after irradiating a laser beam at 3 mW to a portion left to the groove (large width portion of stripe) of the thin film in FIG. 5 at an irradiation rate of 4 m/s. The electron ray diffraction pattern in FIG. 4 is a pattern for a laser irradiation portion and it can be seen from the pattern that the laser irradiated portion of the thin film in FIG. 4 is in an amorphous state. Further, it can be observed from the TEM photograph in FIG. 4 that the laser irradiation portion forms a portion without the fine structure (uniformly black portion) by the reflectivity being lowered, and it is considered that the laser irradiation portion changes from the quasi-stable amorphous state to the amorphous state. It can be seen from the photographs that different two states (stable amorphous state and quasi-stable amorphous state) are actually present in thin films of an identical composition in the amorphous state. However, the two different states described above can not be clearly distinguished so often.
The method of depositing the recording layer so as to form the stable amorphous state can include (1) a method of controlling the temperature of the underlying portion on which the layer is deposited immediately before, immediately after or during deposition of the recording layer to a temperature which makes the recording layer into the stable amorphous state and which is lower than the crystallizing temperature of the recording layer, (2) a method of depositing the recording layer by a laser abrasion method and (3) a method of depositing a film by a sputtering method using helium or neon as a sputtering gas or using a gas mixture containing helium or neon in argon.
The method (1) includes {circle around (1)} a method of heating a substrate or the underlying surface immediately before deposition of the recording layer thereby previously elevating the temperature of the underlying portion for the recording layer, {circle around (2)} a method of starting heating for the substrate or the underlying surface immediately after starting the deposition of the recording layer and continuing the heating during deposition to keep a high temperature for the underlying portion of the recording layer, {circle around (3)} a method of heating the substrate or the deposited surface immediately after the completion of the deposition for the recording layer and {circle around (4)} a method of starting the deposition of the recording layer immediately after the completion of the film deposition taken place just before, utilizing the heat generated by the deposition conducted before the deposition of the recording layer and accumulated in the substrate. The heating method can include a method of irradiating a light including heat rays to the deposited surface of the substrate (underlying portion for the recording layer), a method of heating the substrate holder itself by a heater or the like, heating by radio frequency induction, heating by flash exposure and by plasma treatment.
When the method (1) is adopted in a case where the recording layer comprises a Ge-Te-Sb series alloy and the substrate is made of glass, it is preferred that the substrate temperature during deposition of the recording layer is from 35xc2x0 C. to 150xc2x0 C. That is, the Ge-Te-Sb series alloy forms a stable amorphous state if the deposition temperature is 35xc2x0 C. or higher and causes crystallization if it exceeds 150xc2x0 C. A substrate temperature of 45xc2x0 C. or higher during deposition of the recording layer is preferred since the noise reducing effect in the first recording is remarkably improved. More preferably, it is 55xc2x0 C. or higher. If the substrate is made of a plastic (for example, polycarbonate), it is preferred to control the substrate temperature to 110xc2x0 C. or lower and, more preferably, to 95xc2x0 C. or lower during deposition of the recording layer in order not to cause deformation in the plastic substrate.
The layered structure for the phase change optical recording medium with the relation of the reflectivity being Rc less than Ra can include a constitution, for example, shown in FIG. 6, which comprises, on a substrate 1, a multiple reflection layer 2, a first dielectric layer 3, a recording layer 4, a second dielectric layer 5 and a reflection layer 6 in this order. In FIG. 6, reference numeral 7 denotes an UV-curable resin layer for protecting the surface of the thin film. The multiple reflection layer 2 is a layer for causing multiple reflection of an incident beam between itself and the recording layer 4, and the relation: Rc less than Ra is attained by the presence of the multiple reflection layer 2 within a certain range of a film thickness for the first dielectric layer 3.
The multiple reflection layer 2 can include, for example, a light absorption layer comprising a metal, a semi-metal or a semiconductor, a dielectric multi-layered film formed by alternately laminating or a dielectric material of high refractive index and a dielectric material at low refractivity. The light absorption layer can include, concretely, a layer made of an element comprising Al, Ti, Cr, Ni, Cu, Si, Ge, Ag, Au, Pd, Ga, Se, In, Sn, Sb, Te, Pb, Bi or Ta, or an alloy containing elements selected from the group.
The layered constitution having the dielectric multi-layered film as the multiple reflection layer 2 is preferred since the relation is Rc less than Ra and an optical contrast is also high. The dielectric material of high refractive index in this case is preferably oxides such as TiO2, CeO2 and ZrO2, sulfides such as Sb2S3, CdS and ZnS and nitrides such as Si3N4 and TiN. The dielectric material of low refractive index is preferably, fluorides such as MgF2 and CeF3 and oxides such as SiO2 and Al2O3.
For the first and the second dielectric layers 3 and 5, those materials having high heat resistance and a melting point of 1000xc2x0 C. or higher, for example, SiO2, a mixture of ZnS and SiO2, Al2O3, AlN and Si3N4 are used. Among all, the mixture of ZnS and SiO2 is preferred.
As described previously, since the relation of the reflectivity of the phase change optical recording medium is Rc less than Ra when the multiple reflection layer 2 is disposed and the thickness of the first dielectric layer 3 is within a certain range, the film thickness of the first dielectric layer 3 is defined within a range as capable of providing the relation of the reflectivity: Rc less than Ra in accordance with the entire film constitution.
The film thickness of the second dielectric layer 5 has to be set in view of the recording velocity, the reading velocity and the like. That is, when the linear recording velocity is relatively low, for example, as 6 m/s, the film thickness of the second dielectric layer 5 is preferably made relatively thin to form a phase change optical recording medium of xe2x80x9crapidly cooled structurexe2x80x9d and, specifically, it is preferably from 50 xc3x85 to 500 xc3x85. If it is less than 50 xc3x85, no sufficient recording sensitivity is obtained and, if it is exceeds 500 xc3x85, no sufficient overwrite cyclability can be obtained.
However, in a phase change optical recording medium of xe2x80x9cgradually cooled structurexe2x80x9d in which the film thickness of the velocity dielectric layer 5 is relatively thick, the recording speed can be increased since the recording sensitivity is high. Accordingly, the film thickness of the second dielectric layer 5 may be larger than 500 xc3x85, but it is preferably 3000 xc3x85 or less even for the xe2x80x9cgradually cooled structurexe2x80x9d since generally the read beam is deteriorated incredibly in the case of the thickness being larger than 3000xc3x85.
It has been confirmed by the experiment of the present inventors that if the method (1) is adopted as the method of depositing the recording layer so as to form the stable amorphous state, hydrogen is contained in the first and the second dielectric layers 3 and 5 deposited before and after the recording layer even when a hydrogen gas is not contained in an atmospheric gas during film deposition.
As the material of the recording layer 4, Gexe2x80x94Texe2x80x94Sb series alloys and Gexe2x80x94Texe2x80x94Sbxe2x80x94Bi series alloys are used preferably. Further, the alloys described above may be incorporated, for example, with hydrogen, nitrogen, oxygen, carbon, Al, Ti, Fe, Co, Ni, Cu, Zn, Ga, Se, Sn, In, Ag, Pd, Rh, Ru, Mo, Nb, Hf. Zr, Ta, W, Re, Os, Ir, Pt, Au, Tl or Pb. The elements described above may be incorporated during deposition of the recording layer from a target or by addition in a gaseous state into the atmospheric gas.
The film thickness of the recording layer 4 is desirably from 50 xc3x85 to 1000 xc3x85. No sufficient recording sensitivity can be obtained if it is less than 50 xc3x85, whereas thickness in excess of 1000 xc3x85 is not preferred since this gives rise to a problem in view of the recording sensitivity and the resolution.
As the reflection layer 6 used in the phase-change type recording medium of the constitution described above, a metal, semi-metal or semiconductor is used generally. The film thickness of the reflection layer 6 is preferably 300 xc3x85 or more.
The method of forming each of the layers can include, vapor deposition method, a sputtering method and an ion lating method.